Telemetry event for `early_error`

[Vince Foley]

There are a category of requests that cowboy itself will respond to before it invokes the Plug.Cowboy.Handler, and thus they aren't detectable by any telemetry instrumentation currently in plug or plug_cowboy...

The early_error cowboy callback is implemented, and a subset of requests are logged:

plug_cowboy/lib/plug/cowboy/stream.ex

Lines 27 to 31 in dc2972f

   def early_error(_stream_id, reason, partial_req, resp, _opts) do 
     case reason do 
       {:connection_error, :limit_reached, specific_reason} -> 
         Logger.error(""" 
         Cowboy returned 431 because it was unable to parse the request headers. 

It would be nice to emit a telemetry event for these requests so that they aren't lost. They consume real resources, but are currently invisible to any monitoring solution based on the exiting Plug events.

Thoughts @josevalim?

[José Valim]

What do you have in mind? A single event for early error?

[Vince Foley]

Yes, we won’t get things like duration, but will be able to get minimal visibility into throughout with a single event.

[José Valim]

Sounds good to me!

[Vince Foley]

PR here: #34